The present invention relates to paper coating slips comprising as their binder a copolymer built up from
from 45 to 74.8% by weight of at least one monomer whose homopolymer has a glass transition temperature  less than 20xc2x0 C. (monomers A),
from 25 to 54.8% by weight of at least one monomer whose homopolymer has a glass transition temperature  greater than 50xc2x0 C. (monomers B),
from 0.001 to 1.0% by weight of at least one crosslinking monomer having at least two nonconjugated vinyl groups (monomers C), and optionally
from 0 to 10% by weight of at least one monomer containing acid groups (monomers D), and
from 0 to 10% by weight of at least one further monomer (monomers E).
The invention relates additionally to papers coated with said paper coating slips and to a process for printing these papers.
Paper coating slips consist essentially of a polymeric binder and a white pigment. Coating with paper slips gives untreated papers a smooth white surface. Paper coating slips are intended in particular to bring about an improvement in printability.
The binders used in paper coating slips are, conventionally, acrylate or styrene/butadiene copolymers. Paper coating slips of this kind are described, for example, in WO 97/00776.
It is an object of the present invention to provide paper coating slips having improved properties or alternative paper coating slips having a novel raw-material base.
We have found that this object is achieved by paper coating slips comprising as their binder a copolymer built up from
from 45 to 74.8% by weight of at least one monomer whose homopolymer has a glass transition temperature  less than 20xc2x0 C. (monomers A),
from 25 to 54.8% by weight of at least one monomer whose homopolymer has a glass transition temperature  greater than 50xc2x0 C. (monomers B),
from 0.001 to 1.0% by weight of at least one crosslinking monomer having at least two nonconjugated vinyl groups (monomers C), and optionally
from 0 to 10% by weight of at least one monomer containing acid groups (monomers D), and
from 0 to 10% by weight of at least one further monomer (monomers E).
By the glass transition temperature (Tg) is meant the limiting value approached by the glass transition temperature with increasing molecular weight in accordance with G. Kanig (Kolloid-Zeitschrift and Zeitschrift fxc3xcr Polymere, Vol. 190, page 1, equation 1). The glass transition temperature is determined by the technique of DSC (Differential Scanning Calorimetry, 20 K/min, midpoint measurement, DIN 53765). The Tg values for the homopolymers of the majority of monomers are known and are listed, for example, in Ullmann""s Encyclopedia of Industrial Chemistry, VCH Weinheim, 1992, 5th Edition, Vol. A21, p. 169; further sources of glass transition temperatures of homopolymers are, for example, J. Brandrup, E. H. Immergut, Polymer Handbook, 1st Ed., J. Wiley, New York, 1966, 2nd Ed. J.Wiley, New York, 1975, and 3rd Ed. J. Wiley, New York, 1989.
C1 to C10 alkyl groups hereinbelow are linear or branched alkyl radicals having 1 to 10 carbon atoms, for example, methyl, ethyl, n-propyl, iso-propyl, n-butyl, sec-butyl, iso-butyl, tert-butyl, n-pentyl, iso-pentyl, tert-pentyl, n-hexyl, 2-ethylhexyl, n-nonyl and n-decyl. C5 to C10 cycloalkyl groups are preferably cyclopentyl or cyclohexyl groups, unsubstituted or substituted by 1, 2 or 3 C1 to C4 alkyl groups.
In accordance with the invention, the binder used comprises a copolymer containing in copolymerized form from 45 to 74.8% by weight, preferably from 50 to 65% by weight, based in each case on the copolymer, of at least one monomer A. Accordingly, the copolymer is obtained by polymerizing a monomer mixture comprising said at least one monomer A in an amount of from 45 to 74.8% by weight, preferably from 50 to 65% by weight, based in each case on the overall amount of the monomer mixture. It is noted at this point that the percentage amounts specified in the description for the monomers A to E copolymerized in the copolymer are intended generally to correspond to the amounts of these monomers in the monomer mixture to be polymerized, and vice versa.
Suitable monomers A are preferably vinyl ethers of C3 to C10 alkanols, branched and unbranched C3 to C10 olefins, C1 to C10 alkyl acrylates, C5 to C10 alkyl methacrylates, C5 to C10 cycloalkyl acrylates and methacrylates, C1 to C10 dialkyl maleates and/or C1 to C10 dialkyl fumarates. Particular preference is given to those monomers A whose homopolymers have Tg values  less than 0xc2x0 C. Particularly preferred monomers A are ethyl acrylate, n-propyl acrylate, n-butyl acrylate, iso-butyl acrylate, sec-butyl acrylate, n-hexyl acrylate, 2-ethylhexyl acrylate, n-hexyl methacrylate, 2-ethylhexyl methacrylate, di-n-butyl maleate and/or di-n-butyl fumarate or mixtures thereof.
Suitable monomers B are vinylaromatic monomers, C1 to C4 alkyl methacrylates and/or xcex1,xcex2-unsaturated carbonitriles or dicarbonitriles. They are used in amounts of from 25 to 54.8% by weight and preferably from 35 to 50% by weight, based in each case on the overall amount of the monomer mixture, for the polymerization. Accordingly, the copolymer used in accordance with the invention is built up to the extent of from 25 to 54.8% by weight, and preferably 35 to 50% by weight, of at least one monomer B in copolymerized form. By vinylaromatic monomers are meant in particular derivatives of styrene or of xcex1-methylstyrene in which the phenyl rings are unsubstituted or substituted by 1, 2 or 3 C1 to C4 alkyl groups, chloro and/or methoxy groups. Preference is given to those monomers B whose homopolymers have a glass transition temperature of  greater than 80xc2x0 C. Particularly preferred monomers B are styrene, xcex1-methylstyrene, o- or p-vinyltoluene, methyl methacrylate, acrylonitrile, methacrylonitrile, maleonitrile, fumaronitrile or mixtures thereof.
Said at least one monomer C is used in an amount of from 0.001 to 1% by weight in the monomer mixture, based on its overall amount. Accordingly, the copolymer is built up from 0.001 to 1% by weight of at least one monomer C in copolymerized form. Preferably, the copolymer contains from 0.001 to 0.5% by weight or from 0.001 to 0.1% by weight of copolymerized monomer C. The copolymer frequently contains xe2x89xa70.001% by weight, xe2x89xa70.002% by weight, xe2x89xa70.003% by weight, xe2x89xa70.004% by weight, xe2x89xa70.005% by weight, xe2x89xa70.006% by weight, xe2x89xa70.007% by weight, xe2x89xa70.008% by weight, xe2x89xa70.009% by weight, xe2x89xa70.01% by weight, xe2x89xa70.02% by weight, xe2x89xa70.03% by weight, xe2x89xa70.04% by weight, xe2x89xa70.05% by weight, xe2x89xa70.06% by weight, xe2x89xa70.07% by weight, xe2x89xa70.08% by weight, xe2x89xa70.09% by weight, xe2x89xa70.1% by weight, xe2x89xa70.2% by weight, xe2x89xa70.3% by weight, xe2x89xa70.4% by weight, xe2x89xa70.5% by weight, xe2x89xa70.6% by weight, xe2x89xa70.7% by weight, xe2x89xa70.8% by weight, xe2x89xa70.9% by weight and 1% by weight or  less than 1% by weight, xe2x89xa60.9% by weight, xe2x89xa60.8% by weight, xe2x89xa60.7% by weight, xe2x89xa60.6% by weight, xe2x89xa60.5% by weight, xe2x89xa60.4% by weight, xe2x89xa60.3% by weight, xe2x89xa60.2% by weight, xe2x89xa60.1% by weight, xe2x89xa60.09% by weight, xe2x89xa60.08% by weight, xe2x89xa60.07% by weight, xe2x89xa60.06% by weight, xe2x89xa60.05% by weight, and all values in between, of at least one monomer C in copolymerized form. Monomers suitable as said at least one monomer C are those having at least two nonconjugated ethylenically unsaturated double bonds. Examples thereof are monomers having two vinyl radicals, monomers having two vinylidene radicals, and monomers having two alkenyl radicals. Particularly advantageous in this context are the diesters of dihydric alcohols with xcex1,xcex2-monoethylenically unsaturated monocarboxylic acids, among which acrylic acid and methacrylic acid are preferred. Examples of such monomers having two nonconjugated ethylenically unsaturated double bonds are alkylene glycol diacrylates and dimethacrylates, such as ethylene glycol diacrylate, 1,2-propylene glycol diacrylate, 1,3-propylene glycol diacrylate, 1,3-butylene glycol diacrylate, 1,4-butylene glycol diacrylates and ethylene glycol dimethacrylate, 1,2-propylene glycol dimethacrylate, 1,3-propylene glycol dimethacrylate, 1,3-butylene glycol dimethacrylate, 1,4-butylene glycol dimethacrylate, and also divinylbenzene, vinyl methacrylate, vinyl acrylate, allyl methacrylate, allyl acrylate, diallyl maleate, diallyl fumarate, methylenebisacrylamide, cyclopentadienyl acrylate, triallyl cyanurate, and triallyl isocyanurate.
The copolymer may optionally be obtained by polymerizing a monomer mixture containing up to 10% by weight or from 0.1 to 7% by weight or from 0.5 to 5% by weight, based in each case on the overall monomer amount, of at least one monomer D. Accordingly, the copolymer may contain in copolymerized form up to 10% by weight or from 0.1 to 7% by weight or from 0.5 to 5% by weight of at least one monomer D. It is advantageous if the copolymer contains in copolymerized form from 1 to 4% by weight of monomer D. The monomers D comprise ethylenically unsaturated monomers which are able to form anionic groups. These groups are preferably carboxylate, phosphonate or sulfonate groups, but especially carboxylate groups. Particularly preferred monomers D are monoethylenically unsaturated alkylsulfonic or arylsulfonic acids, such as vinylsulfonic acid, methallylsulfonic acid, vinylbenzenesulfonic acid, acrylamidoethanesulfonic acid, acrylamido-2-methylpropanesulfonic acid, 2-sulfoethyl acrylate, 2-sulfoethyl methacrylate, 3-sulfopropyl acrylate, 3-sulfopropyl methacrylate, and also xcex1,xcex2-ethylenically unsaturated C3 to C6 carboxylic acids, xcex1,xcex2-ethylenically unsaturated C4 to C8 dicarboxylic acids or their anhydrides, such as acrylic acid, methacrylic acid, crotonic acid, fumaric acid, maleic acid, maleic anhydride, itaconic acid and itaconic anhydride, and also the alkali metal salts or ammonium salts of said monomers, especially their sodium salts.
The monomer mixture may optionally likewise contain up to 10% by weight, based on its overall amount, of at least one monomer E. Accordingly, the copolymer optionally contains in copolymerized form up to 10% by weight of at least one monomer E. The amount of monomer E copolymerized in the copolymer may alternatively be from 0.1 to 8% by weight, from 0.2 to 4% by weight, or else from 0.5 to 2% by weight or from 0.5 to 1.5% by weight. Suitable monomers E are monomers having conjugated vinyl groups, such as 1,3-butadiene or isoprene, for example, and also free-radically polymerizable monomers having at least one epoxy group, such as glycidyl acrylate and glycidyl methacrylate, for example, N-alkylol group, such as N-methylolacrylamide and N-methylolmethacrylamide, for example, N-alkyloxy group, such as N-(methoxymethyl)acrylamide and N-(methoxymethyl)methacrylamide for example, and also diacetoneacrylamide, 2-(1-aziridinyl)ethyl methacrylate and, furthermore, amides of xcex1,xcex2-ethylenically unsaturated C3 to C6 carboxylic acids, n-hydroxy-C2-C6alkyl esters of xcex1,xcex2-ethylenically unsaturated C3 to C6 carboxylic acids and/or N-vinyllactams, such as, for example, methacrylic and acrylic C1-C6 hydroxyalkyl esters, such as 2-hydroxyethyl, 3-hydroxypropyl or 4-hydroxybutyl acrylate and methacrylate, and also acrylamide and methacrylamide. Further suitable monomers E are monomers having SiR1R2R3 groups, where R1, R2 and R3 independently of one another are C1 to C4 alkyl or alkoxy groups, such as vinyltrialkoxysilanes, for example, vinyltrimethoxysilane, vinyltriethoxysilane, or acryloyloxy- and methacryloyloxysilanes, for example, xcex3-methacryloyloxypropyltrimethoxysilane and xcex2-methacryloyloxyethyltrimethylsilane. Preferably, the copolymer contains no monomer E in copolymerized form.
The monomers may be polymerized, preferably, free-radically or, where possible, also anionically or cationically. Both free-radical and ionic polymerization are known to the skilled worker as customary polymerization methods.
Free-radical polymerization may be conducted, for example, in solution, for example, in water or an organic solvent (solution polymerization), in aqueous dispersion (emulsion polymerization or suspension polymerization), or in bulk, i.e., substantially in the absence of water or organic solvents (bulk polymerization).
The copolymer used in accordance with the invention is advantageously prepared by means of free-radically initiated aqueous emulsion polymerization.
The free-radically initiated aqueous emulsion polymerization may be conducted, for example, batchwise, with or without the use of seed latices, with the inclusion of all or certain constituents of the reaction mixture in the initial charge, or semicontinuously, preferably with the inclusion of some of the constituents or of certain constituents of the reaction mixture in the initial charge and the subsequent metered addition of the remainder, or by the metering technique without an initial charge. It is also possible for the polymerization to be conducted in stages, with differences in the monomer composition of the individual stages.
In the free-radically initiated aqueous emulsion polymerization the monomers may be polymerized, conventionally, in the presence of a water-soluble initiator and of an emulsifier.
Examples of suitable initiators are sodium, potassium and ammonium peroxodisulfates, tert-butyl hydroperoxide, water-soluble azo compounds, such as 2,2xe2x80x2-azobis(isobutyronitrile), 2,2xe2x80x2-azobis(2,4-dimethylvaleronitrile) and 2,2xe2x80x2-azobis(amidinopropyl)dihydrochloride, for example, or else redox initiators, such as hydrogen peroxide/ascorbic acid, for example. The initiators are frequently used in amounts of from 0.1 to 3% by weight, based on the overall amount of the monomers A to E.
Examples of suitable emulsifiers are alkali metal salts of relatively long-chain fatty acids, alkyl sulfates, alkylsulfonates, alkylated arylsulfonates or alkylated bisphenyl ether sulfonates. Further suitable emulsifiers include reaction products of alkylene oxides, especially ethylene oxide and/or propylene oxide, with fatty alcohols or fatty acids or phenols, and/or alkylphenols, and sulfated derivatives thereof. Emulsifiers are frequently used in amounts of up to 5% by weight, based on the overall amount of the monomers A to E.
In addition to or instead of emulsifiers it is also possible to use natural and/or synthetic protective colloids, such as starch, casein, gelatin, alginates, hydroxycellulose, methylcellulose, carboxymethylcellulose or polyvinyl alcohols, for example.
In the case of aqueous secondary dispersions, the copolymer is first prepared by solution polymerization in an organic solvent and then the solution polymer is dispersed in water, with or without emulsifier. The organic solvent may then be removed by distillation. The preparation of aqueous secondary dispersions is known to the skilled worker and is described, for example, in DE-A 37 20 860.
To adjust the molecular weight of the copolymers it is possible during the polymerization to use compounds known as regulators in amounts of up to 2% by weight, based on the overall amount of the monomers A to E used for the polymerization. Examples of suitable compounds are those containing thiol groups, such as mercaptoethanol, mercaptopropanol, thioglycerol, ethyl thioglycolate, methyl thioglycolate and tert-dodecylmercaptan, for example, and also trichlorobromomethane and allyl alcohols.
The pressure and temperature of the polymerization are of minor importance. In general it is conducted at temperatures between 20 and 200xc2x0 C., preferably at temperatures from 50 to 120xc2x0 C., and with particular preference between 60 and 90xc2x0 C. Advantageously, the free-radically initiated aqueous emulsion polymerization is conducted at atmospheric pressure (1 bar absolute) under an inert gas atmosphere, such as under nitrogen or argon, for example.
Following the polymerization reaction proper it is generally necessary to remove odorous substances, such as residual monomers and other volatile organic constituents, from the aqueous polymer dispersion used in accordance with the invention. This can be done conventionally by physical means, by distillative removal (in particular by way of steam distillation) or by stripping with an inert gas. Furthermore, the amount of residual monomers may be reduced chemically, by free-radical postpolymerization, especially under the action of redox initiator systems, as set out, for example, in DE-A 44 35 423, DE-A 44 19 518, and DE-A 44 35 422, before, during or after the distillative treatment. Especially suitable oxidizing agents for redox-initiated postpolymerization are hydrogen peroxide, tert-butyl hydroperoxide, cumene hydroperoxide, and alkali metal peroxodisulfates. Suitable reducing agents are sodium disulfite, sodium hydrogen sulfite, sodium dithionite, sodium hydroxymethanesulfinate, formamidinesulfinic acid, acetone bisulfite (i.e., the adduct of sodium hydrogen sulfite with acetone), ascorbic acid, and sugar compounds having a reducing action. The postpolymerization with the redox initiator system is conducted in the temperature range from 10 to 100xc2x0 C., preferably at from 20 to 90xc2x0 C. The redox partners may independently of one another be added to the aqueous polymer dispersion completely, in portions, and/or continuously over a period of from 10 minutes to 4 hours. To improve the postpolymerization effect of the redox initiator systems, soluble salts of metals of changing valence, such as iron, copper, or vanadium salts, may also be added to the dispersion. In many cases, complexing agents are added as well, which keep the metal salts in solution under the reaction conditions.
Moreover, the aqueous copolymer dispersions may comprise customary auxiliaries, such as alkali metal hydroxide, ammonia or ethanolamine as neutralizing agents, silicone compounds as defoamers, biozides and also silicone oils or waxes for reducing the tack.
The solids content of the aqueous copolymer dispersion obtained is preferably from 30 to 80% by weight, with particular preference from 45 to 75% by weight.
The number-average diameter of the polymer particles present in the aqueous polymer dispersions, as determined by way of quasielastic light scattering (ISO Standard 13 321), is preferably in the range from 50 to 300 nm, with particular preference in the range from 100 to 200 nm. The polymer particles generally have a monomodal size distribution.
The solution polymerization may be conducted continuously, batchwise or, preferably, semicontinuously by the feed technique. In the latter case, a portion of the monomers A to E may be included in the initial charge to the polymerization vessel, after which this initial charge is heated to the polymerization temperature and the remainder of the monomers are supplied continuously.
Examples of solvents that may be used for the free-radical solution polymerization are water, alcohols, such as iso-propanol or iso-butanol, for example, aromatics, such as toluene or xylene, for example, ethers, such as tetrahydrofuran or dioxane, for example, ketones, such as acetone or cyclohexanone, for example, and esters, such as ethyl acetate or n-butyl acetate, for example.
Preferred initiators include dibenzoyl peroxide, tert-butyl perpivalate, tert-butyl 2-ethylhexanoate, tert-amyl 2-ethylhexyl peroxide, di-tert-butyl peroxide, cumene hydroperoxide, dilauroyl peroxide, didecanoyl peroxide, methyl ethyl ketone peroxide, 2,2xe2x80x2-azobis(isobutyronitrile), 2,2xe2x80x2-azobis(2,4-dimethylvaleronitrile), and 2,2xe2x80x2-azobis(2,3-dimethylbutyronitrile).
In the case of the bulk polymerization, a portion of the polymerization batch comprising monomers and free-radical initiators is generally included in the initial charge, which is heated to the polymerization temperature, and then the remainder is supplied continuously.
Preferably, however, the copolymer is prepared by a free-radically initiated aqueous emulsion polymerization and is used in the form of its aqueous copolymer dispersion.
The glass transition temperature of the copolymer is usually from xe2x88x9240 to +50xc2x0 C., preferably from 0 to +30xc2x0 C., and with particular preference from +5 to +15xc2x0 C.
The paper coating slips comprise the copolymer as binder preferably in amounts of from 1 to 50% by weight, in particular from 5 to 20% by weight, based on the pigment amount (solids/solids).
Normally, the pigments constitute the principal component of the paper coating slips. Frequently used pigments are, for example, natural or precipitated calcium carbonate, kaolin, calcined or aggregated clay, talc, gypsum, titanium dioxide, zinc oxide, barium sulfate, and satin white. Polymer pigments may also be used together with one or more of these inorganic pigments.
Furthermore, the paper coating slips may comprise customary dispersants. Examples of suitable dispersants are polyanions, for example, of polyphosphoric acids or of polyacrylic acids (poly salts), which are present usually in amounts of from 0.1 to 3% by weight, based on the pigment amount.
The paper coating slips may further include what are known as cobinders. Natural cobinders that may be mentioned include starch, casein, gelatin and alginates, hydroxyethylcellulose, methylcellulose and carboxymethylcellulose as modified natural products, and also cationically modified starch. Alternatively, use may be made of customary synthetic cobinders, based for example on vinyl acetate or on acrylate. The amount of cobinder is usually from 0.1 to 10% by weight, based on the pigment amount.
To prepare the paper coating slips, the constituents are mixed in a known way, the copolymer generally being used in the form of an aqueous dispersion.
The water content of the paper coating slip is usually adjusted to from 40 to 75% by weight, based on the solids of the paper coating slip.
The paper coating slip may be applied to the paper substrates by customary methods (cf. Ullmann""s Encyclopxc3xa4die der Technischen Chemie, VCH Weinheim, 1979, 4th Edition, Vol. 17, pages 603 to 609).
Suitable substrate papers are papers of a very wide variety of thicknesses, including cardboard and the like.
The papers are preferably coated at from 2 to 50 g/m2 with the paper coating slip (calculated on a dry basis).
The papers coated with the paper coating slips of the invention exhibit good printability. The papers are especially suitable for offset, gravure or flexographic printing processes.